I. Field of the Invention:
This invention relates generally to the design of cardiac pacemakers, and more particularly to a dual-chamber A-V sequential pacemaker having the capability of automatically adjusting the A-V delay based upon the presence or absence of normally conducted ventricular beats whereby the A-V delay for stimulated beats and the number of normal ventricular beats occurring during intermittent pacing can be optimized.
II. Discussion of the Prior Art:
As is explained in the introductory portion of the Markowitz U.S. Pat. No. 4,421,116, dual chamber pacemakers incorporate means for both stimulating and sensing in both the atrium and ventricle of the heart. The pacemaker is coupled to the heart, via a pacing lead through which sensed atrial and ventricular depolarization signals can pass to the pacemaker circuitry and through which stimulating pulses may be delivered to either or both of the atrium and ventricle. With a dual chamber pacing system, there are four possible modes of pacing as follows:
i. Intrinsic activity sensed in both the atrium and ventricle--no pacing.
ii. Intrinsic activity sensed in atrium and not in ventricle--pace the ventricle.
iii. No intrinsic activity sensed in the atrium and ventricle depolarization sensed--pace the atrium.
iv. No intrinsic activity sensed in either the atrium or ventricle--pace both atrium and ventricle.
A problem exists when a physician attempts to set the A-V delay of the pacemaker because that optimum delay is dependent upon the particular pacing mode in progress. This is attributable to the fact that the propagation time is different for an electrical stimulus to move from the right atrium to the left atrium if a paced beat is involved rather than a normally conducted beat originating at the sinus node. Similarly, it is found to take a greater time for an electrical stimulus to propagate from the right ventricle, i.e., the paced ventricle, to the left ventricle during ventricular pacing than occurs when a normally conducted beat is involved which travels from the A-V node through the bundle of Hiss and the Purkinje fibers. This abnormal deactivation sequence will impair the efficacy of the ventricular contraction. To avoid this situation from occurring, in patients with intrinsic conduction, a long A-V delay, i.e., longer than the P-R interval, would be indicated, to allow the R wave to inhibit the pacemaker. Nevertheless, if a higher degree block develops over time and it becomes necessary to pace the ventricles, a shorter A-V delay would be indicated to optimize the atrial contribution to the ventricular volume.
It is the principal object of the present invention to provide a dual chamber pacer in which the optimum A-V delay is set for paced ventricular beats and still maximizing the probability of sensing intrinsic ventricular contractions before issuing a pacing spike when the pacemaker is operating in one or the other of the atrial synchronous mode or the atrial-ventricular sequential mode, i.e., modes ii and iv, above. A DDD pacemaker is said to be operating in the atrial synchronous mode when the patient's heart has a spontaneous or naturally occurring atrial depolarization at the appropriate time in the heart beat cycle and which is sensed by the atrial sense amplifier in the pacemaker which initiates a time delay, following which a ventricular stimulation pulse will be delivered if no spontaneous ventricular beat is detected. In the atrial-ventricular sequential mode, a spontaneous atrial depolarization (P-wave) does not take place within the appropriate time interval from a preceding ventricular depolarization in accordance with a selected minimum heart rate. In that event, an atrial stimulation pulse is delivered and timing circuitry within the pacemaker establishes an atrial-ventricular time delay interval. Following its completion, ventricular stimulation pulse is delivered unless a spontaneous ventricular depolarization is detected during that interval.
In prior art programmable pacemakers, the A-V delay interval along with various other operating parameters of the pacemaker may be physician selected or programmed by means of an external programming device. Once programmed, the A-V delay interval may be different for atrial synchronous and atrial-ventricular sequential pacing modes. However, this A-V delay is the same whether the patient has normal auriculo-ventricular conduction.
The most efficient ventricular contraction occurs when the atrial stimulus is normally conducted to the ventricles. The restoration of A-V synchrony by artificial pacing in patient's suffering from heart block improves cardiac function and true A-V synchrony. In patients with sick sinus syndrome and/or intermittent A-V block, it is desirable to allow for the normal depolarization of the ventricles whenever possible. This would require the programming of an A-V interval longer than the P-R interval (&gt;150 ms), to allow the R-wave to inhibit the pacemaker. Nevertheless, during the periods where this normal conduction does not take place, it will be required to reduce the A-V delay to its optimum value for paced beats, normally around 150 ms. Thus, naturally occurring beats are preferred. Moreover, the intrinsic conduction also saves battery life. For these reasons, the programming of a long A-V delay seems appropriate to maximize the probability of detecting normally conducted ventricular beats and thereby inhibiting the pacemaker from delivering a pacing pulse. This artificial lengthening of the A-V delay has the unfortunate result that if the patient goes into A-V block, the pacemaker will be pacing with a longer than optimum A-V delay, thereby worsening hemodynamic performance. The programmed A-V delay in a pacemaker has two completely different functions, depending upon whether the patient has been paced in the ventricles or if a normally conducted ventricular beat has been sensed. In the first case, the programmed A-V delay is determining the contribution of the atrial contraction to the ventricular filling, and it should be adjusted in such a way as to maximize the left atrial contribution. In the second case, its function is completely different. Here, it is determining how long the pacemaker should wait before issuing a ventricular pacing spike after a valid atrial event has occurred. As should be obvious to those skilled in the art, a conflict occurs between the two criteria that should be used to establish the A-V delay. Waiting longer than normal to allow for a normally conducted ventricular depolarization to occur will not allow pacing soon enough to compensate for the slowness of the abnormal pathways that the paced depolarization will use to propagate.